1. Field of the Invention
The present invention relates to a liquid crystal display device used in a display part of an electronic apparatus or the like.
2. Description of the related art
FIG. 13 shows the schematic cross-sectional constitution of a conventional liquid crystal display device. As shown in FIG. 13, the liquid crystal display device includes a TFT substrate 102 on which a thin film transistor (TFT) and a pixel electrode are formed for every pixel, and a counter substrate 104 on which color filters (CF) and a common electrode are formed. Both substrates 102, 104 are laminated to each other through a sealing material 152 which is applied to outer peripheral portions of both substrates 102, 104. A mounting terminal 160 which serves to mount a driver IC is formed on the TFT substrate 102. Liquid crystal 106 is sealed between both substrates 102, 104. A cell gap defined between both substrates 102, 104 is held by spherical spacers 146, for example. Further, polarizers 187, 186 are arranged on the outsides which sandwich both substrates 102, 104 therebetween.
FIG. 14 shows the constitution of one pixel of a conventional MVA (Multi-domain Vertical Alignment) type liquid crystal display device. As shown in FIG. 14, the liquid crystal display device includes a plurality of gate bus lines 112 which are formed on the TFT substrate 102, and a plurality of drain bus lines 114 which intersect the gate bus lines 112 through an insulation film. TFTs 120 are formed in the vicinity of positions where the gate bus lines 112 and the drain bus lines 114 intersect each other. In a pixel region which is defined by the gate bus lines 112 and the drain bus lines 114, a pixel electrode 116 is formed. A storage capacitor bus line 118 which extends parallel to the gate bus line 112 is formed in a state that the storage capacitor bus line 118 transverses the pixel region. A storage capacitor electrode 119 is formed over the storage capacitor bus line 118 through an insulation film for every pixel. A linear slit (blank on an electrode) 144 which extends obliquely with respect to polarization axes of the polarizers 186, 187 is formed on the pixel electrode 116. On the counter substrate 104 side, linear projections 142 which extend parallel to the slit 144 are formed. The slit 144 and the linear projections 142 function as the alignment regulating structure which regulates the alignment of the liquid crystal 106.
FIG. 15A and FIG. 15B are cross-sectional views for explaining the alignment regulating structure. In FIG. 15A and FIG. 15B, linear projections 143 are formed on the substrate 102, while the linear projections 142 are formed on the substrate 104. As shown in FIG. 15A, the linear projections 143 on the. TFT substrate 102 side are formed on an alignment film 150 which is formed on the pixel electrode 116, for example. The linear projections 142 of the counter substrate 104 side are formed on an alignment film 151 which is formed on a common electrode 141, for example. At the time of applying no voltage, liquid crystal molecules 108 are aligned substantially normal to surfaces of the substrates. When a voltage is applied between the pixel electrode 116 and the counter electrode 141, as shown in FIG. 15B, the liquid crystal molecules 108 are tilted. Using the linear projections 142, 143 as boundaries, the direction that the liquid crystal molecules 108 are tilted differs and the liquid crystal molecules 108 in a region between the neighboring linear projections 142, 143 are tilted in the same direction. By arranging the alignment regulating structure as shown in FIG. 14, the liquid crystal molecules 108 are tilted in four directions orthogonal to each other in the inside of one pixel. Due to such an alignment division technique, the deviation of a viewing angle which is a phenomenon generated in the liquid crystal display device in which the liquid crystal molecules 108 are tilted only in one direction can be eliminated thus largely improving the viewing angle characteristics. Here, in the constitution shown in FIG. 15A and FIG. 15B, the linear projections 142, 143 are formed on both substrates 102, 104 as the alignment regulating structure. However, there may be a case that the linear projections are formed on only one of the substrates 102, 104. Further, there may be a case that slits are formed on both substrates 102, 104 in place of the linear projections 142, 143. Still further, there may be case that slits 145 are formed only one of substrates 102, 104 as shown in FIG. 16.
FIG. 17 shows one pixel of a general MVA type liquid crystal display device in which slits 144 are formed in a TFT substrate 102 side and linear projections 142 are formed on the counter substrate 104 side. FIG. 18 shows the cross-sectional constitution of the liquid crystal display device taken along a line X-X in FIG. 17, while FIG. 19 shows the cross-sectional constitution of a TFT substrate 102 taken along a line Y-Y in FIG. 17. As shown in FIG. 17 to FIG. 19, a pixel electrode 116 is divided into several regions by slits 144, and the respective regions of the pixel electrode 116 in the inside of the pixel are electrically connected with each other thus maintaining the respective regions at the same potential. The pixel electrode 116 is connected with a source electrode 122 of a TFT 120 via a contact hole 123 and also is electrically connected with a storage capacitor electrode 119 via a contact hole 126. A drain electrode 121 of the TFT 120 is electrically connected with a drain bus line 114. Due to the formation of the slits 144 and linear projections 142, the pixel region is divided into four alignment regions α to δ which differ from each other in the alignment azimuth of liquid crystal molecules 108.
FIG. 20 schematically shows the alignment azimuths of the liquid crystal molecules 108 in the respective alignment regions α to δ and an area ratio of the respective alignment regions in the inside of one pixel. As shown in FIG. 20, areas of the respective alignment regions α to δ which differ in the alignment azimuth of the liquid crystal molecules 108 from each other are set substantially equal in the inside of one pixel. Accordingly, the viewing angle characteristics of the liquid crystal display device do not posses the large dependency on an azimuth angle of a display screen and hence, a favorable display is obtained from any azimuth.
FIG. 21 shows the constitution of one pixel of a liquid crystal display device in which the arrangement of the slits 144 and the linear projections 142 is exchanged with respect to the constitution shown in FIG. 17 to FIG. 19. FIG. 22 shows the cross-sectional constitution of the liquid crystal display device taken along a line Z-Z in FIG. 21. Also in the liquid crystal display device shown in FIG. 21 to FIG. 22, the areas of the alignment regions α to δ are set substantially equal in the inside of one pixel. Accordingly, in the same manner as the liquid crystal display device shown in FIG. 17 to FIG. 19, it is possible to obtain a favorable display from any azimuth.
FIG. 23 is a graph showing the transmissivity characteristics (T-V characteristics) with respect to an applied voltage of a liquid crystal display device adopting a VA (vertically aligned) mode. The applied voltage (V) to a liquid crystal layer is taken on an axis of abscissas and the transmissivity of light is taken on an axis of ordinates. A line L indicates the T-V characteristics in the direction normal to a display screen (hereinafter referred to as “front direction”), and a line M indicates the T-V characteristics in the direction of an azimuth angle of 90° and a polar angle of 60° with respect to the display screen (hereinafter referred to as “tilted direction”). Here, the azimuth angle is an angle which is measured in the counter clock direction using the right direction of the display screen as the difference. Further, the polar angle is an angle which is made with a perpendicular which is erected on the center of the display screen.
As shown in FIG. 23, in the vicinity of a region surrounded by a circle N, a distortion is generated in the change of the transmissivity (brightness). For example, although the transmissivity in the oblique direction is higher than the transmissivity in the front direction at a relatively low gray scale with the applied voltage of approximately 2.5 V, the transmissivity in the tilted direction is lower than the transmissivity in the front direction at a relatively high gray scale with the applied voltage of approximately 3.8 V. As a result, when viewed from the tilted direction, the brightness difference within an effective drive voltage range becomes small. This phenomenon appears most conspicuously in the color change.
FIG. 24A and FIG. 24B show the change of appearance of an image displayed on a display screen. FIG. 24A shows the image as viewed from the front direction, and FIG. 24B shows the image as viewed from the tilted direction. As shown in FIG. 24A and FIG. 24B, when the display screen is viewed from the tilted direction, the color of the image is changed to a whitish color compared to the viewing from the front direction.
FIG. 25A to FIG. 25C show gray scale histograms of three primary colors of red (R), green (G), blue (B) in a reddish image. FIG. 25A shows the gray scale histogram of R, FIG. 25B shows the gray scale histogram of G, and FIG. 25C shows the gray scale histogram of B. In FIG. 25A to FIG. 25C, the gray scales (256 gray scales from 0 to 255) are taken on an axis of abscissas and an existence ratio (%) is taken on an axis of ordinates. As shown in FIG. 25A to FIG. 25C, in the image, R at the relatively high gray scale and G and B at the relatively low gray scales exist at high existence ratios. When such an image is displayed on a display screen of a liquid crystal display device adopting a VA mode and is observed from the tilted direction, R of high gray scale is changed in a relatively dark mode, while G and B of low gray scales are changed in the relatively bright mode. Accordingly, the brightness difference of three primary colors becomes small and hence, the color of the whole screen becomes whitish as viewed from the tilted direction thus giving rise to a drawback that the color reproducibility is lowered.
To overcome the above-mentioned drawback, there has been proposed a following technique. That is, one pixel is divided into a plurality of sub pixels and pixel electrodes which are separated from each other are provided to the respective sub pixels. The respective pixel electrodes establish the electrically capacitive coupling relationship. For example, the pixel electrode of the sub pixel A is directly connected to a source electrode of a TFT, and the pixel electrode of the sub pixel B is connected to the source electrode via a predetermined control capacitance Cc. When the TFT which is formed for every pixel assumes an ON state, a potential is divided according to the capacitance ratio and hence, voltages which differ from each other are applied to the pixel electrodes of the respective sub pixels. Accordingly, the voltage is applied to the liquid crystal layer for every sub pixel. In this manner, when the plurality of sub pixels which differ in the voltage applied to the liquid crystal layer are present in the inside of one pixel, the distortion of the T-V characteristics shown in FIG. 23 is dispersed into the plurality of sub pixels. Accordingly, it is possible to suppress the phenomenon that the image becomes whitish as viewed from the tilted direction and hence, the viewing angle characteristics are improved. Hereinafter, the above-mentioned technique is referred to as a capacitive coupling HT (halftone-gray scale) method.
FIG. 26 shows the constitution of one pixel of a conventional MVA type liquid crystal display device which uses the capacitive coupling HT method. As shown in FIG. 26, a pixel region includes a sub pixel A and a sub pixel B. In the sub pixel A, a pixel electrode 116 is formed on a TFT substrate 102, while in the sub pixel B, a pixel electrode 117 which is separated from the pixel electrode 116 is formed on the TFT substrate 102. The pixel electrode 116 is electrically connected with a storage capacitor electrode 119 and a source electrode 122 of a TFT 120 via contact hole 124. On the other hand, the pixel electrode 117 assumes an electrically floating state. The source electrode 122 is electrically connected with the storage capacitor electrode 119 via a control capacitance electrode 125. The pixel electrode 117 has a region which is overlapped to the control capacitance electrode 125 through a protective film (insulation film) and is indirectly connected with the source electrode 122 by capacitive coupling via a control capacitance Cc which is formed in the region.
Between the pixel electrodes 116, 117, a slit 144 which extends obliquely with respect to an end portion of the pixel region is formed. The slit 144 separates the pixel electrodes 116, 117 from each other and, at the same time, also functions as the alignment regulating structure which regulates the alignment of liquid crystal 106.
A counter substrate 104 which is arranged to face. the TFT substrate 102 in an opposed manner with a liquid crystal layer therebetween includes a common electrode 141(not shown in FIG. 26). A liquid crystal capacitance Clc1 is formed between the pixel electrode 116 of the sub pixel A and the common electrode 141, while a liquid crystal capacitance Clc2 is formed between the pixel electrode 117 of the sub pixel B and the common electrode 141. On the common electrode 141, a linear projection 142 which extends parallel to the slit 144 and functions as the alignment regulating structure is formed. The control capacitance electrode 125 on the TFT substrate 102 side is arranged to be overlapped to the linear projection 142 as viewed in the direction normal to surfaces of the substrates. Further, on the counter substrate 104, a light shielding film (BM) 145 which shields a pixel region end portion from light is formed.
Assume that the TFT 120 is turned on so that a voltage is applied to the pixel electrode 116 and a voltage Vpx1 is applied to the liquid crystal layer of the sub pixel A. Here, since the potential is divided in accordance with the capacitance ratio between the liquid crystal capacitance Clc2 and a control capacitance Cc, a voltage which is different from a voltage applied to the pixel electrode 116 is applied to the pixel electrode 117 of the sub pixel B. A voltage Vpx2 which is applied to the liquid crystal layer of the sub pixel B is expressed as follows.Vpx2=(Cc/(Clc2+Cc))×Vpx1
Here, since a relationship 0<(Cc/(Clc2+Cc))<1 is established, a relationship |Vpx2|<|Vpx1| is established except for Vpx1=Vpx2=0, In this manner, in the liquid crystal display device having the pixel structure shown in FIG. 26, it is possible to make the voltage Vpx1 applied to the liquid crystal layer in the sub pixel A and the voltage Vpx2 applied to the liquid crystal layer in the sub pixel B different from each other in the inside of one pixel and hence, the viewing angle characteristics can be improved.
FIG. 27 shows the constitutions of the pixel electrodes 116, 117 in the vicinity of the slit 144. FIG. 28 shows a display state of the same region as FIG. 27 when the pixel is displayed in white. In FIG. 28, an example of polarization axes 186a, 187a of polarizers 186, 187 is shown together with a region. FIG. 29 schematically shows the alignment of liquid crystal molecules 108 in a region inside a circle P in FIG. 28. As shown in FIG. 27 to FIG. 29, the liquid crystal molecules 108a to which a voltage is applied are respectively tilted normal to the extending direction of the slit 144 and, at the same time, in the directions opposite from each other using the slit 144 (and the linear projection 142) as a boundary. However, although the liquid crystal molecules 108b, 108c in the region above the slit 144 are tilted parallel to the direction that the slit 144 extends, the liquid crystal molecules 108b, 108c are not regulated with respect to the side to which they are tilted. In the region above the slit 144, the liquid crystal molecules 108b which are tilted downwardly in FIG. 29 and the liquid crystal molecules 108c which are tilted upwardly exist and hence, nodes (singular points) 162a, 162b of the liquid crystal alignment are formed. The singular points 162a, 162b are formed at random positions above the slits 144 and hence, the control of the formation positions is difficult. Further, there may be a case that the singular points 162a, 162b are moved along with a lapse of time. Since the spatial and long-period fixing of the formation positions of the singular points 162a, 162b is difficult, after a black display is performed, there may be a case in which the formation positions of the singular points 162a, 162b differ between a state in which white is displayed through an intermediate gray scale display and a state in which white is displayed immediately after the black display. That is, even these white displays are the same, they pass through the different voltage applying histories and hence, they differ in the positions of the singular points 162a, 162b whereby the display becomes different depending on the viewing direction of the screen. Further, when a pressure is locally applied to the display screen with finger pushing or the like and the alignment of the liquid crystal is disturbed, there may be a case that an original alignment state cannot be restored. In this manner, the conventional liquid crystal display device has a drawback that a favorable display quality can not be achieved.
Here, between a drain bus line 114 and the pixel electrodes 116, 117, a given electric capacitance is formed. In the constitution in which an overcoat layer having a large film thickness is not formed as a layer between the drain bus line 114 and the pixel electrodes 116, 117, due to the difference in distance within a substrate plane between the drain bus line 114 and the pixel electrodes 116, 117, a value of the formed electric capacitance is liable to be easily changed. Accordingly, when a relative patterning displacement is generated with respect to the drain bus line 114 and the pixel electrodes 116, 117 due to shot irregularities which occur when the division exposure is performed or the like, for example, in the manufactured liquid crystal display device, the display irregularities which differ in display characteristics for every division exposure region are observed with naked eyes. Accordingly, it is necessary to separate end portions of the pixel electrodes 116, 117 from the drain bus line 114 as much as possible so as to make the difference in display characteristics hardly visible even when the patterning displacement occurs. However, when the end portions of the pixel electrodes 116, 117 are separated from the drain bus line 114, regions where the pixel electrodes 116, 117 are formed become narrow and hence, an aperture ratio of the pixel is lowered thus giving rise to a drawback that the brightness is lowered.
Further, in laminating the TFT substrate 102 and the counter substrate 104 to each other, there may arise a given lamination displacement. Accordingly, it is necessary to make an opening region of a BM 145 formed on the counter substrate side narrower than the regions where the pixel electrodes 116, 117 are formed on the TFT substrate 102 side. Accordingly, there arises a drawback that the aperture ratio of the pixels and the brightness are further lowered.    [Patent document 1] Japanese Patent Laid-open Publication H-2(1990)-12    [Patent document 2] U.S. Pat. No. 4,840,460    [Patent document 3] Japanese Patent Publication 3076938